Recently, as a result of the achievement of high density and large capacity in optical recording media, the demand for optical recording media tends to significantly grow year by year. Conventionally, the optical recording medium is classified into three types according to the type of use including the type which is solely used for reading, the type which allows additional recording (write once-type), and the type which allows rewriting of information, respectively.
Of these, the write once-type optical recording medium records information by applying such processes consisting essentially of the projection of laser beams onto the recording medium of the optical recording medium and the occurrence of fusion or decomposition of the beam-projected spot for providing geometric pits in the beam-projected recording medium.
Besides the above example, information can also be recorded in the write once-type optical recording medium by applying those processes consisting essentially of the projection of laser beams onto the recording medium of the optical recording medium and the variation of optical characteristics like the index of reflection of the beam-projected portion by crystalizing or forming a non-crystalline condition in the beam-projected portion so that information can be recorded.
Conventionally, there is such a rewritable optical recording medium in which information is recorded making use of a magneto-optial effect or a phase transition of the recording medium. Information is recorded by using the optical recording medium applying a magneto-optical effect by sequentially executing those processes described below. First, laser beams are projected onto a recording medium containing films which are magnetized in a direction perpendicular to the surface of the recording medium. Next, by applying a magnetic field from an external source, the magnetized direction of the magnetized portion is inverted from that of the portion in which is not exposed to the projection of beams in the condition where the beam-projected portion is heated above the Curie temperature, thus allowing the rewritable optical recording medium to be recorded information.
While information is recorded by using the optical recording medium applying a phase transition by executing those processes described below.
First, laser beams are projected onto the recording medium, and then, a phase of the beam-projected portion of the recording medium is converted into a crystalline condition from the non-crystalline condition or vice versa, thereby recording information in the optical recording medium. Note that any of those methods mentioned above for recording information in the optical recording medium use laser beams as a heating source, and thus, these processes can be summarized as "heat-mode" recording systems.
Recently, independent of the "heat-mode" recording system, research and development of such an optical recording medium using the "photon-mode" recording system has been conducted. As a typical example of the "photon-mode" recording system, such a system using organic compounds presenting photochromic phenomenon is conventionally known. For example, as was disclosed by the Japanese Laid-Open Patent Publication No. 62-165751 (1987), information is recorded on the multiplex basis according to the wavelength dimensions and the polarization-degree dimensions on a recording spot of laser beams by initially writing information in a cumulative film of a cumulated plurality of monomolecular films containing organic coloring matter on recording medium by projecting beams having different wavelengths or degree of polarization, followed by varying either the wavelengths or the degree of polarization of the laser beams used for recording.
The photochromic phenomenon is the phenomenon in which a certain solid or liquid material reversibly varies in color by projecting a light. There are a wide variety of organic compounds which exhibit the a photochromic phenomenon such as hydrazone, osazone, stilbene, salicylaldehyde, spiropyrane, fulgide, azobenzene, and derivatives of these, for example. Out of these organic compounds, a typical example of an optical recording medium using a well-known recording medium composed of fulgide is described below. By projection of ultraviolet rays having a wavelength of about 340 nm and visible rays, fulgide alternately causes intramolecular ring closure as shown in FIG. 3 (b) and intramolecular ring opening as shown in FIG. 3 (a). As a result, as is well known, variation of the absorption spectrum shown by the solid line and the assumptive line in FIG. 4 reversibly occurs.
For example, the absorption spectrum shown by solid line in FIG. 4 is initially caused by preliminarily projecting visible rays onto fulgide, followed by execution of the recording by projecting recording beams having a wavelength of about 340 nm thereon. The portion exposed to the projection of the recording beam varies into the state indicated by the absorption spectrum shown by the assumptive line in FIG. 4. As a result, for example, when this portion is subjected to the projection of weak beams having a wavelength of about 350 nm close to the wavelength of the recording beam, a certain difference in absorbance shown by .DELTA.T in FIG. 4 is produced between the portion subjected to the projection of the recording beam and the portion free from the beam projection, and thus, based on the difference in absorbance, a recorded signal can be read.
On the other hand, in order to erase the recorded signal, the absorption spectrum shown by solid line I in FIG. 4 restores itself as a result of the projection of beams having a wavelength of about 500 nm onto the portion subjected to the projection of the recording beam. As mentioned above, since the photochromic phenomenon is reversibly presented, the recording medium can be used for an erasable optical recording medium.
In the manner mentioned above, recording information in the optical recording medium making use of a photochromic phenomenon is implemented by projecting laser beams onto the recording medium. On the other hand, the spot diameter of beams focused by the optical lens is constrained by the diffraction, where the spot diameter is confined to the scope of the wavelength. Actually, the spot diameter of visible rays or near-infrared rays is confined to about one micron. When executing digital recording of one-bit information in the beam-spot region, any of those conventional optical recording media is merely provided with a maximum of 10.sup.8 bit/cm.sup.2 of recording density, and as a result, sufficient recording density cannot be provided for any conventional optical recording medium.
In order to more densely record information using a photon-mode recording system, recently, research and development of newer optical recording media making use of a photochemical hole burning phenomenon are underway. Nevertheless, actually, there are still a variety of technical problems to solve before offering them for practical use.